Method of manufacturing substrates for semiconductor devices, corresponding substrate and semiconductor device

ABSTRACT

A pre-molded substrate includes a sculptured, electrically conductive laminar structure having spaces therein. The laminar structure includes a die pad having a first die pad surface configured to mount a semiconductor chip. A pre-mold material molded onto the laminar structure penetrates into the spaces and provides a laminar pre-molded substrate with the first die pad surface left exposed. The peripheral edge of the die pad includes an alternation of first and second anchoring formations to the pre-mold material. The first anchoring formations counter first detachment forces inducing displacement of the die pad with respect to the pre-mold material in a first direction from the second die pad surface to the first die pad surface. The second anchoring formations counter second detachment forces inducing displacement of the die pad with respect to the pre-mold material in a second direction from the first die pad surface to the second die pad surface.

PRIORITY CLAIM

This application claims the priority benefit of Italian Application forPatent No. 102021000020114, filed on Jul. 28, 2021, the content of whichis hereby incorporated by reference in its entirety to the maximumextent allowable by law.

TECHNICAL FIELD

The description relates to semiconductor devices.

One or more embodiments can be applied to semiconductor power devicesfor the automotive sector, for instance.

BACKGROUND

In substrates such as pre-molded leadframes, adequate adhesion betweenthe sculptured, electrically conductive structure of the leadframe(copper, for instance) and the pre-mold resin (an epoxy resin, forinstance) molded thereon should desirably absorb stresses generated ifthe pre-molded leadframe is pressed or bent.

Particularly, pads in pre-molded leadframes should desirably resistpressing forces (as developed, e.g., during ribbon ultrasonic wedgebonding) as well as pulling forces (as developed, e.g., during ribbonpulling for second bond, or as a result thermo-mechanical stress underoperation).

It is noted that, while advantageous for other purposes, slot-likeanchoring structures provide limited pulling resistance while taking anon-negligible pad area.

There is a need in the art to deal with the issues as discussed in theforegoing.

SUMMARY

One or more embodiments relate to a method.

One or more embodiments relate to a corresponding substrate (leadframe)for semiconductor devices.

One or more embodiments relate to a semiconductor device.

One or more embodiments provide a die pad design for a pre-moldedleadframe (formed through standard half-etch before pre-molding, by aleadframe supplier, for instance) comprising an alternation of‘fingernail-like’ anchoring structures on the top and bottom sides ofthe die pad.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments will now be described, by way of example only,with reference to the annexed figures, wherein:

FIG. 1 is exemplary of a conventional substrate such as a pre-moldedleadframe and of forces that may be applied to such a leadframe;

FIG. 2 is exemplary of a similar substrate provided with slot-likeanchoring structures;

FIGS. 3A and 3B are cross-sectional along line II-II of FIG. 2 showinghow a substrate as illustrated in FIG. 2 can resist opposite forcesapplied thereto;

FIG. 4 is a perspective view of a part of the structure of a substratesuch as a pre-molded leadframe according to embodiments of the presentdescription;

FIG. 5 is a cross-sectional view along lines V-V in FIG. 4 ; and

FIGS. 6 and 7 are view substantially corresponding to the view of FIG. 5showing possible variants of embodiments of the present description.

DETAILED DESCRIPTION

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated.

The figures are drawn to clearly illustrate the relevant aspects of theembodiments and are not necessarily drawn to scale.

The edges of features drawn in the figures do not necessarily indicatethe termination of the extent of the feature.

In the ensuing description, various specific details are illustrated inorder to provide an in-depth understanding of various examples ofembodiments according to the description. The embodiments may beobtained without one or more of the specific details, or with othermethods, components, materials, etc. In other cases, known structures,materials, or operations are not illustrated or described in detail sothat various aspects of the embodiments will not be obscured.

Reference to “an embodiment” or “one embodiment” in the framework of thepresent description is intended to indicate that a particularconfiguration, structure, or characteristic described in relation to theembodiment is comprised in at least one embodiment. Hence, phrases suchas “in an embodiment”, “in one embodiment”, or the like, that may bepresent in various points of the present description do not necessarilyrefer exactly to one and the same embodiment. Furthermore, particularconfigurations, structures, or characteristics may be combined in anyadequate way in one or more embodiments.

The headings/references used herein are provided merely for convenienceand hence do not define the extent of protection or the scope of theembodiments.

Semiconductor devices may comprise one or more semiconductor chips ordice arranged (attached) on substrates such as leadframes.

Plastic packages are commonly used for semiconductor devices. Suchpackages may include a leadframe providing a base substrate comprisingelectrically conductive material such as copper, sized and shaped toaccommodate semiconductor chips or dice and providing pad connections(leads) for these chips or dice.

The designation “leadframe” (or “lead frame”) is currently used (see,for instance the USPC Consolidated Glossary of the United States Patentand Trademark Office) to indicate a metal frame that provides supportfor an integrated circuit chip or die as well as electrical leads tointerconnect the integrated circuit in the die or chip to otherelectrical components or contacts.

Leadframes are conventionally created using technologies such as aphoto-etching technology. With this technology, metal (e.g., copper)material in the form of a foil or tape is etched on the top and bottomsides to create various pads and leads.

Substrates such as leadframes are advantageously provided in apre-molded version wherein an insulating resin (an epoxy resin, forinstance) fills the empty spaces between the die pads and leads.

A pre-molded leadframe is a thus a laminar substrate that issubstantially flat with the pre-mold material (resin) filling the spacesin the electrically conductive structure (metal material such as copper,for instance) of the leadframe, that has been bestowed a sculpturedappearance including empty spaces during forming, by etching, forinstance.

The total thickness of the pre-mold leadframe is the same thickness ofthe sculptured electrically conductive structure.

During the assembly process of semiconductor devices using a pre-moldedleadframe, a pre-molded leadframe can be exposed to repeated stress.

Particularly, pads in pre-molded leadframes are exposed to pressingforces (as developed, e.g., during ribbon ultrasonic wedge bonding) aswell as to pulling forces (as developed, e.g., during ribbon pulling forsecond bond, or as a result of thermo-mechanical stress underoperation).

FIG. 1 is a cross-sectional view of a portion of a pre-molded leadframeillustrated as comprising, in general, electrically conductive (metal,e.g., copper) portions 10 included in a sculptured, electricallyconductive structure of the leadframe (not visible in its entirety),having spaces filled by the pre-mold material (resin) 12.

A pre-molded leadframe PLF as illustrated in FIG. 1 has opposite firstand second die pad surfaces 10A and 10B, with the first surfaces 10Aconfigured to have at least one semiconductor chip mounted thereon.

FIG. 1 is thus exemplary of an approach wherein a sculptured,electrically conductive laminar structure is provided having spacestherein, the laminar structure including one or more die pads 10 havinga first die pad surface 10A configured to have at least onesemiconductor chip mounted thereon as well as a second die pad surface10B opposite the first die pad surface 10A.

FIG. 1 is likewise exemplary of an approach wherein pre-mold material 12is molded onto the laminar structure 10. The pre-mold material 12penetrates into the spaces formed (e.g., etched) in the sculptured,electrically conductive laminar structure and provides a laminarpre-molded substrate PLF 10, 12 including one or more die pads leftexposed by the pre-mold material 12 at the first surface 10A with theperiphery of the die pad or pads 10 bordering on the pre-mold material12 molded onto the laminar structure.

As illustrated in FIG. 1 , the sculpturing bestowed on the electricallyconductive (metal, e.g., copper) portions 10 of the (pre-molded)leadframe is beneficial in keeping all the leadframe parts (leads anddie pads) together in a robust structure to facilitate the subsequentprocess steps.

These steps may include, for instance—after attaching one or more chipsor dice on the die pad or pads in the leadframe 10, 12 and the chips ordice being electrically bonded to the leads in the leadframe (notexplicitly visible in FIG. 1 )—a molding resin being molded to providean insulating encapsulation of a final device.

FIG. 1 is exemplary of a leadframe for, e.g., a Quad-Flat No-leads (QFN)device where the sculptured, electrically conductive structure of theleadframe is half etched, that is, a part of copper material is removed,e.g., at the periphery of die pad so that (as visible in FIG. 1 ) thedie pad is larger at the front or top surface 10A of the leadframe thanat the back or bottom surface 10B of the leadframe.

Half etching can be performed in any manner known to those of skill inthe art.

Also, while “half” etching is currently referred to for simplicity, thepart of copper material removed does not necessarily correspond to halfthe thickness of the metal structure of the leadframe.

It is noted that the description above also applies—mutatis mutandis—tothe embodiments discussed in the following, e.g., in connection withFIGS. 4 to 7 . Such a detailed description will not be repeated forbrevity.

The die pad being larger at the front or top surface 10A than at theback or bottom surface 10B as illustrated in FIG. 1 increases themolding adhesion around the leadframe parts. This is due to a step-likeinterface formed between the conductive (metal, e.g., copper) portions10 of the leadframe and the pre-mold material (resin) 12 molded thereon.

After the pre-mold material is solidified (e.g., by thermosetting, asotherwise conventional in the art) this design results in increasedresistance to detachment (delamination) between the conductive portions10 of the leadframe and the non-conductive pre-mold material 12 moldedthereon as possibly induced by “pulling” forces F1 (namely forces urgingthe metal part 10 shown in FIG. 1 in the direction from the back orbottom surface 10B towards the front or top surface 10A) and by“pushing” or “pressing” forces F2 (namely forces urging the metal part10 shown in FIG. 1 in the direction from the front or top surface 10Atowards the back or bottom surface 10B).

Such a step-like interface includes undercuts as indicated at 120 wherethe periphery of the conductive portion 10 of the leadframe abutsagainst the pre-mold material (resin) 12. This provides a form couplingsuch that the resistance to “pushing” forces F2 (directed downwards inFIG. 1 ) is inevitably (much) higher than the resistance to “pulling”forces F1 (directed upwards in FIG. 1 ).

Forces applied to a pre-molded leadframe such as 10, 12 in FIG. 1 duringthe assembly flow of a semiconductor device include both pressing forcessuch as, e.g., pressing forces applied during ribbon bonding by abonding tool together with ultrasonic vibrations and pulling forces,e.g., when a ribbon is pulled or cut by moving or opening a bondingcutting tool.

An arrangement as illustrated in FIG. 1 , with the undercuts 120contrasting mainly the pressing forces (e.g., F2) and exhibiting pooradhesion resistance to pulling forces (e.g., F1) cannot be regarded assatisfactory for a variety of practical applications.

FIG. 2 and FIGS. 3A and 3B illustrate a solution as disclosed in UnitedStates Patent Application Publication No. 2021/0193591 (to which EP 3840 040 A1 corresponds) assigned to the same Assignee of the presentapplication.

The leadframe of United States Patent Application Publication No.2021/193591 A1 comprises a die pad portion having a first planardie-mounting surface 10A and a second planar surface 10B opposed thefirst surface 10A.

As visible in FIG. 2 (where the conductive structure 10 of the leadframeis shown prior to molding the pre-mold material 12) the die pad surfaces10A and 10B have facing peripheral rims jointly defining a peripheraloutline of the die pad. At least one cavity 100 is provided extendingthrough the die pad from the first planar surface 10A to the secondplanar surface 10B to define an anchoring portion of the die padpositioned between said at least one cavity and the peripheral outline.

A first etched part extends into the first planar die-mounting surface10A to a first depth less than a thickness of the die pad and a secondetched part extends into the second planar surface to a second depthless than the thickness of the die pad. The first etched part defines astep surface within the cavity 100 that extends parallel to the firstplanar die-mounting surface 10A and the second etched part defines athickness of the anchoring portion which is less than the thickness ofthe die pad.

FIGS. 3A and 3B (where the pre-mold material 12 is visible, filling thespaces in the sculptured, electrically conductive structure 10 of theleadframe) show that such an arrangement may lead to the formations ofundercuts 120, 120′ facing in opposite directions.

These undercuts 120 and 120′ provide a form coupling of the electricallyconductive structure 10 of the leadframe and the pre-mold material 12providing improved resistance also to pulling forces F1 (FIG. 3A) inaddition is pushing or pressing forces F2 (FIG. 3B).

Here again, however, the resistance to pushing forces F2 may end up bybeing higher than the resistance to pulling forces F1, while for certainapplications having a resistance to pulling forces F1 equal or possiblyhigher than the resistance to pushing or pressing forces F2 may be adesirable feature.

In any case, cavities/apertures such as 100 in FIGS. 2, 3A and 3Bsubtract area that should be desirably left available for dieattachment.

In FIGS. 4 to 7 , parts or elements like parts or elements alreadydiscussed in connection with the previous figures are indicated withlike reference symbols, so that a detailed description will not berepeated for brevity.

Examples as presented in FIGS. 4 to 7 comprise, in the place of ahalf-etched step-like metal-to-resin interface (as illustrated in FIG. 1) or slots (such as 100 in FIG. 2 ), an alternation or series of (e.g.,fingernail-like) cutaway portions 200A, 200B formed along the border(that is, along the peripheral edge) of the die pad 10, advantageouslyall around the die pad 10. Each cutaway portion is formed by ahalf-etched slot arranged at (and extending in from) the peripheral edgeof the die pad.

These cutaway portions 200A, 200B, that are arranged alternatively(possibly alternately) at the front or top surface 10A and at the backor bottom surface 10B, are filled by the pre-mold resin 12 creating(once the resin is solidified, e.g., via thermosetting) a robuststructure of the pre-molded leadframe PLF.

The cutaway portions 200A, 200B may be all equal in shape (e.g., with asame length in the direction of the edges the die pad 10).

The cutaway portions 200A, 200B may be provided equal in number at thefront or top surface 10A and at the back or bottom surface 10B, so theresistance and the resin adhesion is balanced in both directions (forcesF1 and F2).

The provision of the cutaway portions 200A, 200B does not entail anyappreciate reduction of the surface (indicated as DAS in FIG. 4 )available for die attachment (and possibly for the provision ofassociated ribbons or wires) at the front surface 10A of the leadframe.

As visible, e.g., in FIG. 4 the top or front surface of the die paddesignated DAS is exempt from any aperture such as the slot 100 in FIG.2 .

It is noted that in the perspective view of FIG. 4 the conductivestructure 10 of the leadframe is shown prior to molding the pre-moldmaterial 12, with also some of the leads of the leadframe, indicated10′, visible on the right-hand side of FIG. 4 .

Examples as presented in FIGS. 4 to 7 comprise at and along theperipheral edge of the pad 10 an alternation of first anchoringformations 200A and second anchoring formations 200B that anchor the pad10 to the pre-mold material 12 thus providing (once the material 12 issolidified, e.g., via thermosetting) a robust structure of thepre-molded leadframe PLF.

The first anchoring formations 200A are configured to counter “pulling”detachment forces, namely forces such as F1 inducing displacement of thedie pad 10 with respect to the pre-mold material 12 in a first direction(upwards in the figures) from the second die pad surface 10B to thefirst die pad surface 10A.

The second anchoring formations 200B are configured to counter “pushing”or “pressing” detachment forces, namely forces such as F2 inducingdisplacement of the die pad 10 with respect to the pre-mold material 12in a second direction (downwards in the figures) from the first die padsurface 10A to the second die pad surface 10A.

As illustrated herein, the first anchoring formations 200A are providedat the first die pad surface 10A and the second anchoring formations200B are provided at the second die pad surface 10B.

While other shapes (e.g., protrusions) are possible, providing theanchoring formations 200A and 200B as cutaway portions of the peripheraledge of the die pad 10 is advantageous in so far as the pre-moldmaterial 12 can penetrate into these cutaway portions at the peripheraledge of the die pad 10 and establish (once solidified) a strong bondkeeping together the various portions of the leadframe PLF.

Irrespective of the specific implementation details, a good degree offlexibility exists in providing an alternation of anchoring formations200A and 200B along the peripheral edge at one or more of the sides of adie pad such as the die pad 10 illustrated herein.

As exemplified in FIG. 5 the alternation of first anchoring formations200A and second anchoring formations 200B may comprise single firstanchoring formations 200A alternating (interleaved) with single secondanchoring formations 200B.

That is, the alternation as exemplified in FIG. 5 comprises the sequenceof a first formation 200A, a second formation 200B, a first formation200A, a second formation 200B, and so on.

As exemplified in FIGS. 6 and 7 the alternation of first anchoringformations 200A and second anchoring formations 200B may comprises atleast one single first anchoring formation 200A alternating with aplurality of second anchoring formations 200B.

For instance: the alternation as exemplified in FIG. 6 comprises thesequence of three first formations 200A followed by a second formation200B; and the alternation as exemplified in FIG. 7 comprises thesequence of three second formations 200B followed by a first formation200A.

While not expressly illustrated for brevity, the alternation maycomprise plural first formations 200A interleaved with plural secondformation 200B.

For instance (this is just one possible example) the alternation maycomprise the sequence of three first formations 200A followed by twosecond formations 200B, in turn followed by three first formations 200Aagain followed by two second formations 200B, and so on.

Such interleaving may also comprise different numbers of first andsecond formations at each iteration.

For instance (again, this is just one possible example) the alternationmay comprise the sequence of three first formations 200A followed by twosecond formations 200B, in turn followed by two first formations 200Afollowed by three second formations 200B, and so on.

This flexibility may be advantageously relied upon to “adjust” asdesired the resistance of the leadframe PLF to pulling forces andpushing or pressing forces.

This may possibly take into account the characteristics of thesemiconductor chips or dice intended to be mounted (attached) on theleadframe PLF. In FIGS. 5 to 7 the outline of a semiconductor chip ordie C mounted onto the die pad 10 is shown in dashed line.

For instance, providing in the alternation equal numbers of firstanchoring formations 200A and second anchoring formations 200B (see,e.g., FIG. 5 ) facilitates making the laminar pre-molded substrate PLFequally resistant to pulling forces F1 and to pushing or pressing forcesF2.

Providing in the alternation first anchoring formations 200A higher innumber than the second anchoring formations 200B (see FIG. 6 )facilitate making the laminar pre-molded substrate PLF more resistant topulling forces F1 than to pushing or pressing forces F2.

Providing in the alternation second anchoring formations 200B higher innumber than the first anchoring formations 200A (see FIG. 7 ) facilitatemaking the laminar pre-molded substrate PLF more resistant to pushing orpressing forces F2 than to pulling forces F1.

Options as exemplified in FIGS. 6 and 7 may be helpful in dealing withsemiconductor chips or dice C mounted on the die pad surface 10A leftexposed by the pre-mold material 12 that are warped. This may be thecase of large and/or thin semiconductor chips or dice C that may exhibit“crying” or “smiling” shapes.

Without prejudice to the underlying principles, the details andembodiments may vary, even significantly, with respect to what has beendescribed in the foregoing, by way of example only, without departingfrom the extent of protection.

The claims are an integral part of the technical teaching providedherein with reference to the embodiments.

The extent of protection is determined by the annexed claims.

1. A method, comprising: providing a sculptured electrically conductivelaminar structure having spaces therein, the sculptured electricallyconductive laminar structure including at least one die pad having afirst die pad surface configured to mount a semiconductor chip as wellas a second die pad surface opposite the first die pad surface; andmolding pre-mold material to penetrate into said spaces of thesculptured electrically conductive laminar structure and provide alaminar pre-molded substrate including said first die pad surface leftexposed by the pre-mold material with the peripheral edge of the atleast one die pad bordering on the pre-mold material molded onto thesculptured electrically conductive laminar structure; wherein providingthe sculptured electrically conductive laminar structure comprisesproviding at the peripheral edge of the at least one die pad analternation of: first anchoring formations of the at least one die padto the pre-mold material, the first anchoring formations configured tocounter first detachment forces inducing displacement of the at leastone die pad with respect to the pre-mold material in a first directionfrom the second die pad surface to the first die pad surface; and secondanchoring formations of the at least one die pad to the pre-moldmaterial, the second anchoring formations configured to counter seconddetachment forces inducing displacement of the at least one die pad withrespect to the pre-mold material in a second direction from the firstdie pad surface to the second die pad surface.
 2. The method of claim 1,wherein the first anchoring formations are located at the first die padsurface and the second anchoring formations are located at the seconddie pad surface.
 3. The method of claim 1, wherein each of the first andthe second anchoring formations is provided as a cutaway portion of theperipheral edge of the at least one die pad, wherein the pre-moldmaterial molded onto the sculptured electrically conductive laminarstructure penetrates into said cutaway portions at the peripheral edgeof the at least one die pad.
 4. The method of claim 1, wherein saidalternation of first anchoring formations and second anchoringformations comprises at least two first anchoring formations alternatingwith at least two second anchoring formations.
 5. The method of claim 1,wherein said alternation of first anchoring formations and secondanchoring formations comprises a single first anchoring formationalternating with a single second anchoring formation.
 6. The method ofclaim 1, further comprising providing in said alternation equal numbersof first anchoring formations and second anchoring formations, whereinthe laminar pre-molded substrate is equally resistant to said firstdetachment forces and to said second detachment forces.
 7. The method ofclaim 1, further comprising providing in said alternation firstanchoring formations greater in number than said second anchoringformations, wherein the laminar pre-molded substrate is more resistantto said first detachment forces than to said second detachment forces.8. The method of claim 1, further comprising providing in saidalternation second anchoring formations greater in number than saidfirst anchoring formations, wherein the laminar pre-molded substrate ismore resistant to said second detachment forces than to said firstdetachment forces.
 9. A substrate, comprising: a sculptured electricallyconductive laminar structure having spaces therein, the sculpturedelectrically conductive laminar structure including at least one die padhaving a first die pad surface configured to mount a semiconductor chipmounted thereon as well as a second die pad surface opposite the firstdie pad surface; and pre-mold material molded onto the sculpturedelectrically conductive laminar structure, wherein the pre-mold materialpenetrates into said spaces and provides a laminar pre-molded substrateincluding said first die pad surface left exposed by the pre-moldmaterial with the periphery of the at least one die pad bordering on thepre-mold material molded onto the sculptured electrically conductivelaminar structure; wherein along the peripheral edge of the at least onedie pad there is provided an alternation of: first anchoring formationsof the at least one die pad to the pre-mold material, the firstanchoring formations configured to counter first detachment forcesinducing displacement of the at least one die pad with respect to thepre-mold material in a first direction from the second die pad surfaceto the first die pad surface; and second anchoring formations of the atleast one die pad to the pre-mold material, the second anchoringformations configured to counter second detachment forces inducingdisplacement of the at least one die pad with respect to the pre-moldmaterial in a second direction from the first die pad surface to thesecond die pad surface.
 10. The substrate of claim 9, wherein the firstanchoring formations are provided at the first die pad surface andwherein the second anchoring formations are provided at the second diepad surface.
 11. The substrate of claim 9, wherein each of the first andthe second anchoring formations comprises a cutaway portion of theperipheral edge of the at least one die pad, wherein the pre-moldmaterial molded onto the sculptured electrically conductive laminarstructure penetrates into said cutaway portions of the peripheral edgeof the at least one die pad.
 12. The substrate of claim 9, wherein saidalternation of first anchoring formations and second anchoringformations comprises at least two first anchoring formations alternatingwith at least two second anchoring formations.
 13. The substrate ofclaim 9, wherein said alternation of first anchoring formations andsecond anchoring formations comprises a single first anchoringformations alternating with a single second anchoring formation.
 14. Thesubstrate of claim 9, wherein said alternation comprises equal numbersof first anchoring formations and second anchoring formations, whereinthe laminar pre-molded substrate is equally resistant to said firstdetachment forces and to said second detachment forces.
 15. Thesubstrate of claim 9, wherein said alternation comprises first anchoringformations greater in number than said second anchoring formations,wherein the laminar pre-molded substrate is more resistant to said firstdetachment forces than to said second detachment forces.
 16. Thesubstrate of claim 9, wherein said alternation comprises secondanchoring formations greater in number than said first anchoringformations, wherein the laminar pre-molded substrate is more resistantto said second detachment forces than to said first detachment forces.17. A semiconductor device, comprising: a substrate according to claim9; and a semiconductor chip mounted on the at least one die pad surfaceleft exposed by the pre-mold material.